The present invention relates to a method of transferring target substrates in a semiconductor processing system, particularly, to a method of transferring semiconductor wafers from a cassette (container) into a boat for a heat treatment. The term "semiconductor processing" used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or an LCD substrate, by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
In the manufacturing process of a semiconductor device, a vertical heat treating system is employed, so that a heat treatment such as oxidation or diffusion is applied in a single step to a plurality of semiconductor wafers. FIG. 8 schematically shows a vertical heat treating system of this type.
In carrying out a heat treatment by using the vertical heat treating system, wafers W held in a cassette 11 are transferred by a transfer apparatus 12 into a wafer boat 13. Then, the boat 13 is loaded by a boat elevator 14 into a vertical furnace 15 for applying a predetermined heat treatment to the wafers W. After the heat treatment, the wafers W are transferred from the boat 13 back into the cassette 11. Then, untreated wafers W are transferred from another cassette 11 into the boat 13 for applying a heat treatment to these wafers W.
The inner wall of the cassette 11 is provided with a plurality of grooves (not shown), into which the peripheral portions of the wafers W are inserted, such that a plurality of wafers W can be held in a stacked state with a distance therebetween within the cassette 11. On the other hand, the boat 13 has, e.g., three support rods provided with grooves (not shown), into which the peripheral portions of the wafers are inserted, such that a large number of wafers W can be held in a stacked state with a distance therebetween within the boat 13. The number of wafers W which can be held in the boat 13 is larger than that of wafers held in the cassette 11.
The transfer apparatus 12 comprises a plurality of transfer arms 12a, e.g., five transfer arms. These five transfer arms can be moved back and forth, up and down and can be swung simultaneously. Also, one of the five arms 12a, e.g., the central arm 12a, can be moved back and forth independent of the other 4 arms 12a. Since 25 wafers are generally held in a single cassette 11, the transfer apparatus 12 is provide with the five arms 12a, so that all the wafers held in the cassette 11 can be taken out by the five arms 12a without bringing about a fractional number of wafers.
In transferring the wafers W from the cassette 11 into the boat 13, the five arms 12a of the transfer apparatus 12 are moved into positions immediately under the wafers W which are to be held by these arms 12a, respectively. Then, the transfer apparatus 12 is moved up to cause each of the arms 12a to pick up the wafers W. Then, the arms 12a are moved backward, and the transfer apparatus 12 is swung and moved in a vertical direction to cause the arms 12a holding the wafers W to face a predetermined position of the boat 13. Then, the arms 12a are moved forward so as to cause the peripheral portions of the wafers W to be inserted into the grooves formed in the support rods of the boat 13. Then, the transfer apparatus 12 is moved down so as to deliver the wafers W to the boat 13.
In recent years, the semiconductor device is made finer and finer, and it is important to more effectively suppress formation of a spontaneous oxide film and generation of particles. To meet this requirement, a closed type cassette including a lid, which permits maintaining a high cleanliness within the cassette, is widely used nowadays in place of a conventional open type cassette. In general, the closed type cassette is constructed to house 13 wafers.
Where the transfer apparatus 12 provided with the five arms 12a is used for transferring wafers W from the closed type cassette into the boat 13, it is customary to employ the following transfer method. Specifically, the method combines a step of moving the five arms 12a back and forth simultaneously to hold five wafers and another step of moving only one of these five arms back and forth to hold a single wafer W. For example, the step of simultaneously moving five arms is performed twice, with the step of moving only one arm is performed three times, so as to transfer 13 wafers held in the closed type cassette into the boat 13 by five moving steps.
It is also known to the art to use a transfer apparatus provided with four transfer arms. In this case, these four transfer arms are simultaneously moved back and fourth three times, and only one transfer arm is moved back and forth once, so as to transfer 13 wafers held in the closed type cassette into the boat 13 by four moving steps.
The time required for transferring the wafers from the cassette 11 into the boat 13 is increased with an increase in the number of moving steps of a transfer apparatus. It should be noted that the wafers must be transferred between the cassette 11 and the boat 13 both before and after the heat treatment. Accordingly, the total time required for transferring the wafers for the heat treatment is twice the time required for the one way transfer, leading to a low through-put of the heat treatment.